This invention relates to an electromagnetic print head, comprising a plurality of magnetic circuits each of which comprises an armature supported by a spring, a permanent magnet for biasing the armature in one direction, and an electromagnet which can be energized so as to cancel the biasing force of the permanent magnet, so that print means fixed to the armature may be driven by energizing the electromagnet and thereby driving the armature with the restoring force of the spring, and in particular to such a printer which is adapted to high speed printing and can produce uniform print pressure.
A dot matrix printer of this type, which is sometimes called as a spring charge type printer, is shown for instance in U.S. Pat. No. 4,389,127 issued June 21, 1983. A printer of this type typically includes 9, 16 or 24 sets of print means or print wires and magnetic circuits, and is widely used for printing alphanumeric characters and Japanese characters as dot matrices.
In such a printer, it is necessary to have a number of independent magnetic circuits to achieve selective energization of the print means such as print wires for dot matrix printing and, since the mass of inertia of the print head is desired to be minimized for rapid reciprocating motion of the print head, the size of the print head is desired to be minimized.
Therefore, the structure is often so cramped that some interferences between the magnetic circuits are unavoidable to a certain extent. The magnetic interferences between magnetic circuits may cause some fluctuations in the electromagnetic force for a certain magnetic circuit in canceling the magnetic force of the permanent magnet depending on the states of the magnetic circuits neighboring thereto and this can be a cause of the unevenness of the print speed or the print pressure of the print means of this particular magnetic circuit.
Therefore, conventionally, it has been believed that such interferences or cross talks between magnetic circuits in one print head are generally undesirable primarily because such interferences have been believed to be generally responsible for uneven print pressures, and in actual design of a print head it has been customary to minimize the magnetic interferences between magnetic circuits to the possible extent permitted by other design considerations
Based on such recognition, the Inventor has conducted various experimental studies on the effect of such magnetic interferences between magnetic circuits of a print head particularly on print speed and print quality, and discovered that by making the magnetic couplings between neighboring magnetic circuits uneven in a certain manner the print speed can be increased without causing any substantial unevenness in print pressure.